Calculate and plot (BOX option) the Mandelbrot distribution
                                               
1     FUN2 10 mandel.f [1] -2.4 .8 [1] -1.2 1.2 ' '                                                              
2     HI/PL 10 BOX                                                                                               

FORTRAN Routine MANDEL
                                                                                    
      real function mandel(xp)                                                                                          
3     dimension xp(2)                                                                                            
      data nmax/30/                                                                                                     
      x=xp(1)                                                                                                           
      y=xp(2)                                                                                                           
      xx=0.                                                                                                             
      yy=0.                                                                                                             
      do n=1,nmax                                                                                                       
         tt=xx*xx-yy*yy+x                                                                                               
         yy=2.*xx*yy+y                                                                                                  
         xx=tt                                                                                                          
         if (4..lt.xx*xx+yy*yy) go to 20                                                                                
      enddo                                                                                                             
   20 mandel=float(n)/float(nmax)                                                                                       
      end                                                                                                               

1. This example shows one of the usages of COMIS. In this case, the name of the function to be plotted by FUN2 is replaced by a COMIS FORTRAN function.
2. CHOPT=' ' in the command FUN2 means to fill only the histogram without producing the plot which is by default a surface. The plot is produced by the command HIST/PLOT .
3. The vector XP is an input parameter given by FUN2 , for each cell, to the FORTRAN program. XP contains the X and Y coordinates of each cell. You can try to insert:

                                                                                                                      
         print*, XP                                                                                                        
   

   in mandel.f to see the values changing (in this case it is better to set the input parameter of the macro to 10).